The Vatican is raising the possibility that the conclave to elect the next pope might start earlier than March 15, the earliest date possible under current rules that require a 15-20 day waiting period after the papacy becomes vacant.

Vatican spokesman the Rev Federico Lombardi said top officials can study the Holy See's constitution to determine whether such a rule change is possible. The 15-20 day rule is in place to allow time for the arrival in Rome of "all those (cardinals) who are absent".

But Lombardi noted that the cardinals already know that this pontificate will end on February 28 and can get to Rome in plenty of time. He said the Vatican rules are open to interpretation and that "this is a question that people are discussing".